Method and apparatus for imaging nonstationary object

ABSTRACT

Disclosed is an optical imaging system, and associated method, comprising a stage module configured to support an object such that an area of the object is illuminated by an illumination beam; an objective lens configured to collect at least one signal beam, the at least one signal beam originating from the illuminated area of the object; an image sensor configured to capture an image formed by the at least one signal beam collected by the objective lens; and a motion compensatory mechanism operable to compensate for relative motion of the stage module with respect to the objective lens during an image acquisition. The motion compensatory mechanism causes a compensatory motion of one or more of: said objective lens or at least one optical element thereof; said image sensor; and/or an optical element comprised within a detection branch and/or illumination branch of the optical imaging system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 20210884.1 which wasfiled on Dec. 1, 2020 and which is incorporated herein in its entiretyby reference.

FIELD

The present invention relates to methods and apparatuses for imagingnonstationary objects, and in particular such methods and apparatuses inrelation to metrology applications in the manufacture of integratedcircuits.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×λ/NA,where λ is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

Metrology tools are used in many aspects of the IC manufacturingprocess, for example as alignment tools for proper positioning of asubstrate prior to an exposure, leveling tools to measure a surfacetopology of the substrate, for e.g., focus control and scatterometrybased tools for inspecting/measuring the exposed and/or etched productin process control. In each case, a positioner or stage module may berequired to accurately position a substrate support which holds asubstrate. In these applications, optical measurement or inspection overmore than one area of a substrate is often desired. For example, overlayis typically measured over multiple overlay targets or marks located atdifferent positions on a wafer for each process layer. After a currentoverlay target is measured, the positioner moves the wafer with respectto the position of an objective lens such that a next target ispositioned under the objective lens and aligned with an illuminationbeam that is focused by the objective lens.

The Move-Acquire-Measure time (MAM) is the time that is taken to ‘Move’the wafer from the current overlay target to the next overlay target,‘Acquire’ an image of the next overlay target, and ‘Measure’ or computean overlay value. The MAM determines throughput of a metrology orinspection tool. According to the current state of the art, due tocompromise with increased size, complexity and cost of e.g., fasterstage platform, a large part of the ‘Move’ time is spent on decelerationand acceleration of moving masses (e.g., a heavy substrate support, orthe image sensor) to attain a completely or substantially stationarytarget with respect to imaging optics of the tool (e.g., objective lens,image sensor) during image acquisition. A non-stationary target willlead to a distorted (or blurry) image. Since acquired images aretypically used to compute or determine the value of a parameter ofinterest (e.g., overlay value), an image with poor quality will resultin loss of measurement performance (e.g., a lower measurement accuracyor reproducibility). The time needed to transition between a fast movingstate and a stationary state is a significant part of the MAM time, andtherefore this is limiting throughput. Increasing throughput at givenstage complexity or reducing stage complexity at same throughput is anobjective of the present invention.

SUMMARY

In a first aspect of the invention there is provided an optical imagingsystem, comprising: a stage module configured to support an object suchthat an area of the object is illuminated by an illumination beam; anobjective lens configured to collect at least one signal beam, the atleast one signal beam originating from the illuminated area of theobject; an image sensor configured to capture an image formed by the atleast one signal beam collected by the objective lens; and a motioncompensatory mechanism operable to compensate for relative motion of thestage module with respect to the objective lens during an imageacquisition by causing a compensatory motion of one or more of: saidobjective lens or at least one optical element thereof; said imagesensor; and/or an optical element comprised within a detection branchand/or illumination branch of the optical imaging system.

In a second aspect of the invention there is provided a method forimaging an object using an optical imaging system, comprising:illuminating an area of the object with an illumination beam; collectingat least one signal originating from the illuminated area of the objectduring an acquisition period during at least a portion of which saidobject is non-stationary; acquiring an image from the at least onesignal beam on an image sensor; and performing a compensatory motion ofan optical element of the optical imaging system during said acquisitionperiod to compensate for relative motion of the object with respect toan objective lens module used to collect the at least one signal duringthe acquisition period such that the image is maintained atsubstantially the same position on the image sensor during theacquisition period.

Other aspects of the invention comprise metrology device comprising theoptical system of the second aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic overview of a scatterometry apparatus used asa metrology device, which may comprise a radiation source according toembodiments of the invention;

FIG. 5 depicts a schematic overview of a level sensor apparatus whichmay comprise a radiation source according to embodiments of theinvention;

FIG. 6 depicts a schematic overview of an alignment sensor apparatuswhich may comprise a radiation source according to embodiments of theinvention;

FIG. 7 shows two diagrams illustrating respectively the moving speed ofa sample as a function of time for the cases where image acquisition isperformed while the sample is completely stationary (top diagram), andfor the cases where image acquisition is performed while the sample isstill in motion (bottom diagram).

FIG. 8 depicts schematically an embodiment of a scatterometer metrologytool in a first configuration;

FIG. 9 depicts schematically operating principle of motion compensationimplemented in the first configuration of the scatterometer metrologytool in accordance with an embodiment;

FIG. 10 depicts schematically an embodiment of the scatterometermetrology tool in a second configuration;

FIG. 11 depicts schematically an embodiment of the scatterometermetrology tool in a third configuration;

FIG. 12 is a block diagram illustrating training of CNN to predictcorrected images from blurred images and from the known blurring kernel;

FIG. 13 is a block diagram illustrating training of a CNN to predictoverlay values for x and y directly from blurred images and from theknown blurring kernel;

FIG. 14 is a block diagram illustrating an inference of a trained CNNpredicting an unblurred image from a motion-blurred input image plusmotion blur kernel; and

FIG. 15 is a block diagram illustrating an inference of a trained CNNpredicts overlay for x and y directly from a motion-blurred input imagepair plus motion blur kernel; and

FIG. 16 depicts a block diagram of a computer system for controlling abroadband radiation source.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

<Reticle>

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1 ) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3 . One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device) —typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Tools to make such measurement are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT. Scatterometers areversatile instruments which allow measurements of the parameters of alithographic process by having a sensor in the pupil or a conjugateplane with the pupil of the objective of the scatterometer, measurementsusually referred as pupil based measurements, or by having the sensor inthe image plane or a plane conjugate with the image plane, in which casethe measurements are usually referred as image or field basedmeasurements. Such scatterometers and the associated measurementtechniques are further described in patent applications US20100328655,US2011102753A1, US20120044470A, US20110249244, US20110026032 orEP1,628,164A, incorporated herein by reference in their entirety.Aforementioned scatterometers may measure gratings using light from softx-ray and visible to near-IR wavelength range.

In a first embodiment, the scatterometer MT is an angular resolvedscatterometer. In such a scatterometer reconstruction methods may beapplied to the measured signal to reconstruct or calculate properties ofthe grating. Such reconstruction may, for example, result fromsimulating interaction of scattered radiation with a mathematical modelof the target structure and comparing the simulation results with thoseof a measurement. Parameters of the mathematical model are adjusteduntil the simulated interaction produces a diffraction pattern similarto that observed from the real target.

In a second embodiment, the scatterometer MT is a spectroscopicscatterometer MT. In such spectroscopic scatterometer MT, the radiationemitted by a radiation source is directed onto the target and thereflected or scattered radiation from the target is directed to aspectrometer detector, which measures a spectrum (i.e. a measurement ofintensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile of the target givingrise to the detected spectrum may be reconstructed, e.g. by RigorousCoupled Wave Analysis and non-linear regression or by comparison with alibrary of simulated spectra.

In a third embodiment, the scatterometer MT is an ellipsometricscatterometer. The ellipsometric scatterometer allows for determiningparameters of a lithographic process by measuring scattered radiationfor each polarization states. Such metrology apparatus emits polarizedlight (such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationSer. Nos. 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968,12/922,587, 13/000,229, 13/033,135, 13/533,110 and 13/891,410incorporated herein by reference in their entirety.

In one embodiment of the scatterometer MT, the scatterometer MT isadapted to measure the overlay of two misaligned gratings or periodicstructures by measuring asymmetry in the reflected spectrum and/or thedetection configuration, the asymmetry being related to the extent ofthe overlay. The two (typically overlapping) grating structures may beapplied in two different layers (not necessarily consecutive layers),and may be formed substantially at the same position on the wafer. Thescatterometer may have a symmetrical detection configuration asdescribed e.g. in co-owned patent application EP1,628,164A, such thatany asymmetry is clearly distinguishable. This provides astraightforward way to measure misalignment in gratings. Furtherexamples for measuring overlay error between the two layers containingperiodic structures as target is measured through asymmetry of theperiodic structures may be found in PCT patent application publicationno. WO 2011/012624 or US patent application US 20160161863, incorporatedherein by reference in its entirety.

Other parameters of interest may be focus and dose. Focus and dose maybe determined simultaneously by scatterometry (or alternatively byscanning electron microscopy) as described in US patent applicationUS2011-0249244, incorporated herein by reference in its entirety. Asingle structure may be used which has a unique combination of criticaldimension and sidewall angle measurements for each point in a focusenergy matrix (FEM—also referred to as Focus Exposure Matrix). If theseunique combinations of critical dimension and sidewall angle areavailable, the focus and dose values may be uniquely determined fromthese measurements.

A metrology target may be an ensemble of composite gratings, formed by alithographic process, mostly in resist, but also after etch process forexample. Typically the pitch and line-width of the structures in thegratings strongly depend on the measurement optics (in particular the NAof the optics) to be able to capture diffraction orders coming from themetrology targets. As indicated earlier, the diffracted signal may beused to determine shifts between two layers (also referred to ‘overlay’)or may be used to reconstruct at least part of the original grating asproduced by the lithographic process. This reconstruction may be used toprovide guidance of the quality of the lithographic process and may beused to control at least part of the lithographic process. Targets mayhave smaller sub-segmentation which are configured to mimic dimensionsof the functional part of the design layout in a target. Due to thissub-segmentation, the targets will behave more similar to the functionalpart of the design layout such that the overall process parametermeasurements resembles the functional part of the design layout better.The targets may be measured in an underfilled mode or in an overfilledmode. In the underfilled mode, the measurement beam generates a spotthat is smaller than the overall target. In the overfilled mode, themeasurement beam generates a spot that is larger than the overalltarget. In such overfilled mode, it may also be possible to measuredifferent targets simultaneously, thus determining different processingparameters at the same time.

Overall measurement quality of a lithographic parameter using a specifictarget is at least partially determined by the measurement recipe usedto measure this lithographic parameter. The term “substrate measurementrecipe” may include one or more parameters of the measurement itself,one or more parameters of the one or more patterns measured, or both.For example, if the measurement used in a substrate measurement recipeis a diffraction-based optical measurement, one or more of theparameters of the measurement may include the wavelength of theradiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US2016-0161863 and published USpatent application US 2016/0370717A1 incorporated herein by reference inits entirety.

A metrology apparatus, such as a scatterometer, is depicted in FIG. 4 .It comprises a broadband (white light) radiation projector 2 whichprojects radiation onto a substrate 6. The reflected or scatteredradiation is passed to a spectrometer detector 4, which measures aspectrum 10 (i.e. a measurement of intensity as a function ofwavelength) of the specular reflected radiation. From this data, thestructure or profile giving rise to the detected spectrum may bereconstructed by processing unit PU, e.g. by Rigorous Coupled WaveAnalysis and non-linear regression or by comparison with a library ofsimulated spectra as shown at the bottom of FIG. 3 . In general, for thereconstruction, the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. Such a scatterometer may beconfigured as a normal-incidence scatterometer or an oblique-incidencescatterometer.

Overall measurement quality of a lithographic parameter via measurementof a metrology target is at least partially determined by themeasurement recipe used to measure this lithographic parameter. The term“substrate measurement recipe” may include one or more parameters of themeasurement itself, one or more parameters of the one or more patternsmeasured, or both. For example, if the measurement used in a substratemeasurement recipe is a diffraction-based optical measurement, one ormore of the parameters of the measurement may include the wavelength ofthe radiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in US patent application US2016/0161863 and published USpatent application US 2016/0370717A1 incorporated herein by reference inits entirety.

Another type of metrology tool used in IC manufacture is a topographymeasurement system, level sensor or height sensor. Such a tool may beintegrated in the lithographic apparatus, for measuring a topography ofa top surface of a substrate (or wafer). A map of the topography of thesubstrate, also referred to as height map, may be generated from thesemeasurements indicating a height of the substrate as a function of theposition on the substrate. This height map may subsequently be used tocorrect the position of the substrate during transfer of the pattern onthe substrate, in order to provide an aerial image of the patterningdevice in a properly focus position on the substrate. It will beunderstood that “height” in this context refers to a dimension broadlyout of the plane to the substrate (also referred to as Z-axis).Typically, the level or height sensor performs measurements at a fixedlocation (relative to its own optical system) and a relative movementbetween the substrate and the optical system of the level or heightsensor results in height measurements at locations across the substrate.

An example of a level or height sensor LS as known in the art isschematically shown in FIG. 5 , which illustrates only the principles ofoperation. In this example, the level sensor comprises an opticalsystem, which includes a projection unit LSP and a detection unit LSD.The projection unit LSP comprises a radiation source LSO providing abeam of radiation LSB which is imparted by a projection grating PGR ofthe projection unit LSP. The radiation source LSO may be, for example, anarrowband or broadband light source, such as a supercontinuum lightsource, polarized or non-polarized, pulsed or continuous, such as apolarized or non-polarized laser beam. The radiation source LSO mayinclude a plurality of radiation sources having different colors, orwavelength ranges, such as a plurality of LEDs. The radiation source LSOof the level sensor LS is not restricted to visible radiation, but mayadditionally or alternatively encompass UV and/or IR radiation and anyrange of wavelengths suitable to reflect from a surface of a substrate.

The projection grating PGR is a periodic grating comprising a periodicstructure resulting in a beam of radiation BE1 having a periodicallyvarying intensity. The beam of radiation BE1 with the periodicallyvarying intensity is directed towards a measurement location MLO on asubstrate W having an angle of incidence ANG with respect to an axisperpendicular (Z-axis) to the incident substrate surface between 0degrees and 90 degrees, typically between 70 degrees and 80 degrees. Atthe measurement location MLO, the patterned beam of radiation BE1 isreflected by the substrate W (indicated by arrows BE2) and directedtowards the detection unit LSD.

In order to determine the height level at the measurement location MLO,the level sensor further comprises a detection system comprising adetection grating DGR, a detector DET and a processing unit (not shown)for processing an output signal of the detector DET. The detectiongrating DGR may be identical to the projection grating PGR. The detectorDET produces a detector output signal indicative of the light received,for example indicative of the intensity of the light received, such as aphotodetector, or representative of a spatial distribution of theintensity received, such as a camera. The detector DET may comprise anycombination of one or more detector types.

By means of triangulation techniques, the height level at themeasurement location MLO can be determined. The detected height level istypically related to the signal strength as measured by the detectorDET, the signal strength having a periodicity that depends, amongstothers, on the design of the projection grating PGR and the (oblique)angle of incidence ANG.

The projection unit LSP and/or the detection unit LSD may includefurther optical elements, such as lenses and/or mirrors, along the pathof the patterned beam of radiation between the projection grating PGRand the detection grating DGR (not shown).

In an embodiment, the detection grating DGR may be omitted, and thedetector DET may be placed at the position where the detection gratingDGR is located. Such a configuration provides a more direct detection ofthe image of the projection grating PGR.

In order to cover the surface of the substrate W effectively, a levelsensor LS may be configured to project an array of measurement beams BE1onto the surface of the substrate W, thereby generating an array ofmeasurement areas MLO or spots covering a larger measurement range.

Various height sensors of a general type are disclosed for example inU.S. Pat. Nos. 7,265,364 and 7,646,471, both incorporated by reference.A height sensor using UV radiation instead of visible or infraredradiation is disclosed in US2010233600A1, incorporated by reference. InWO2016102127A1, incorporated by reference, a compact height sensor isdescribed which uses a multi-element detector to detect and recognizethe position of a grating image, without needing a detection grating.

Another type of metrology tool used in IC manufacture is an alignmentsensor. A critical aspect of performance of the lithographic apparatusis therefore the ability to place the applied pattern correctly andaccurately in relation to features laid down in previous layers (by thesame apparatus or a different lithographic apparatus). For this purpose,the substrate is provided with one or more sets of marks or targets.Each mark is a structure whose position can be measured at a later timeusing a position sensor, typically an optical position sensor. Theposition sensor may be referred to as “alignment sensor” and marks maybe referred to as “alignment marks”.

A lithographic apparatus may include one or more (e.g. a plurality of)alignment sensors by which positions of alignment marks provided on asubstrate can be measured accurately. Alignment (or position) sensorsmay use optical phenomena such as diffraction and interference to obtainposition information from alignment marks formed on the substrate. Anexample of an alignment sensor used in current lithographic apparatus isbased on a self-referencing interferometer as described in U.S. Pat. No.6,961,116. Various enhancements and modifications of the position sensorhave been developed, for example as disclosed in US2015261097A1. Thecontents of all of these publications are incorporated herein byreference.

FIG. 6 is a schematic block diagram of an embodiment of a knownalignment sensor AS, such as is described, for example, in U.S. Pat. No.6,961,116, and which is incorporated by reference. Radiation source RSOprovides a beam RB of radiation of one or more wavelengths, which isdiverted by diverting optics onto a mark, such as mark AM located onsubstrate W, as an illumination spot SP. In this example the divertingoptics comprises a spot mirror SM and an objective lens OL. Theillumination spot SP, by which the mark AM is illuminated, may beslightly smaller in diameter than the width of the mark itself.

Radiation diffracted by the alignment mark AM is collimated (in thisexample via the objective lens OL) into an information-carrying beam IB.The term “diffracted” is intended to include zero-order diffraction fromthe mark (which may be referred to as reflection). A self-referencinginterferometer SRI, e.g. of the type disclosed in U.S. Pat. No.6,961,116 mentioned above, interferes the beam IB with itself afterwhich the beam is received by a photodetector PD. Additional optics (notshown) may be included to provide separate beams in case more than onewavelength is created by the radiation source RSO. The photodetector maybe a single element, or it may comprise a number of pixels, if desired.The photodetector may comprise a sensor array.

The diverting optics, which in this example comprises the spot mirrorSM, may also serve to block zero order radiation reflected from themark, so that the information-carrying beam IB comprises only higherorder diffracted radiation from the mark AM (this is not essential tothe measurement, but improves signal to noise ratios).

Intensity signals SI are supplied to a processing unit PU. By acombination of optical processing in the block SRI and computationalprocessing in the unit PU, values for X- and Y-position on the substraterelative to a reference frame are output.

A single measurement of the type illustrated only fixes the position ofthe mark within a certain range corresponding to one pitch of the mark.Coarser measurement techniques are used in conjunction with this toidentify which period of a sine wave is the one containing the markedposition. The same process at coarser and/or finer levels may berepeated at different wavelengths for increased accuracy and/or forrobust detection of the mark irrespective of the materials from whichthe mark is made, and materials on and/or below which the mark isprovided. The wavelengths may be multiplexed and de-multiplexedoptically so as to be processed simultaneously, and/or they may bemultiplexed by time division or frequency division.

In this example, the alignment sensor and spot SP remain stationary,while it is the substrate W that moves. The alignment sensor can thus bemounted rigidly and accurately to a reference frame, while effectivelyscanning the mark AM in a direction opposite to the direction ofmovement of substrate W. The substrate W is controlled in this movementby its mounting on a substrate support and a substrate positioningsystem controlling the movement of the substrate support. A substratesupport position sensor (e.g. an interferometer) measures the positionof the substrate support (not shown). In an embodiment, one or more(alignment) marks are provided on the substrate support. A measurementof the position of the marks provided on the substrate support allowsthe position of the substrate support as determined by the positionsensor to be calibrated (e.g. relative to a frame to which the alignmentsystem is connected). A measurement of the position of the alignmentmarks provided on the substrate allows the position of the substraterelative to the substrate support to be determined.

Metrology tools MT, such as a scatterometer, topography measurementsystem, or position measurement system mentioned above, as well as manyother optical inspection tools may use a positioner or stage module forprecisely and accurately positioning a sample (e.g., a semiconductorsubstrate or wafer) with respect to an optical beam. Depending onapplications, the positioner may be configured to enable movement withmultiple degrees of freedom (e.g., six degrees of freedom). Driven bythe growing demand of manufacturing modern integrated circuits with evershrinking sizes, metrology and inspection tools that can offer higherresolution and better reliability are under a fast-paced and extensivedevelopment. In many existing metrology and inspection tools, opticalresolution is enhanced by using a high NA objective lens. For example,in existing scatterometer metrology tools for overlay measurement, anobjective lens having a high NA (e.g., between 0.7 and 0.97) is oftenused. Where measurements are conducted on an image of the sample, simplyusing an objective lens with a high resolving power may not necessarilyguarantee a high measurement accuracy. This is because in those casesmeasurement accuracy also relies on quality of acquired images. Anydistorted or blurry image would significantly lower measurementaccuracy.

As described in the Background section, in order to avoid generation ofdistorted or blurry images, a positioner carrying a sample (e.g., asubstrate or wafer) should be completely or substantially stationarybefore image acquisition can start. The time needed to transitionbetween a fast moving state and a stationary state accounts for asignificant part of the MAM time. A large amount of such transitioningtime is spent on deceleration and acceleration of e.g., a heavysubstrate support to ensure a completely or substantially stationarysample with respect to imaging optics of the tool (e.g., objective lens,image sensor) during image acquisition. Since the MAM time is inverselyproportional to the throughput of a metrology or inspection tool, i.e.,the number of samples processed per unit time, it is thus desirable toreduce the MAM in order to boost the throughput.

FIG. 7 shows two diagrams illustrating the moving speed of a sample as afunction of time respectively for the cases where image acquisition isperformed while the sample is completely stationary (FIG. 7(a)), and forthe cases where image acquisition is performed while the sample is stillin motion (FIG. 7(b)). It should be appreciated that FIG. 7(b) can alsobe used for illustrating the cases where image acquisition is performedwhile an optical component (e.g., objective) of an optical system ismoving with respect to a stationary sample. With reference to FIG. 7(a),the MAM time in this particular case comprises a first duration of timeΔT1 taken for moving a sample between two positions (e.g., a currentposition and a next position), and a second duration of time ΔT2 takenfor acquiring an image. The first duration of time ΔT1 is defined as theduration between the time a positioner starts to move from the currentposition and the time the positioner settles completely (or the speed iszero) in the next position. With reference to FIG. 7(b), the MAM timecomprises a first duration of time ΔT1′ taken for moving a samplebetween two imaging areas and a second duration of time ΔT2′ taken foracquiring an image. However, due to the nature of non-stationary sample,the first duration of time T1′ is defined as the duration between thetime a positioner leaves a current imaging area (at a certain non-zerospeed) and the time enters a next imaging area (at a certain non-zerospeed). In comparison to the top diagram, the movement time ΔT1′ in thebottom diagram is reduced as the sample is allowed to be imaged while itis still in motion with a non-zero speed. Assuming acquisition time isconstant (using the same image sensor), i.e. ΔT2=ΔT2′, the reduction inthe MAM time achievable by imaging a nonstationary sample is thereforethe difference between the two transitioning periods, i.e. ΔT1 and ΔT1′.

Taking an image on a non-stationary sample or with a non-stationaryimage sensor often results in degraded image quality (e.g., blurryimages). Various techniques have been developed and adopted in existingoptical systems for motion compensation and image enhancement. Forexample, optical pick-up units are typically used in Digital Video Disc(DVD) and Blu-ray drives in laptops. Those optical pick-up units usevery compact voice-coil motors (VCM) in combination with wire springs toaccurately control radial, focus and tilt movements of a high NAobjective lens (e.g., NA=0.85 for Blu-ray).

Optical image stabilization (OIS) techniques have been employed in manydigital single-lens reflex (DSLR) cameras where a mechanical X-Yvoice-coil actuator is used to control a separate, movable lens elementor lens group, and/or an image sensor (e.g., a complementarymetal-oxide-semiconductor (CMOS) sensor). The voice-coil actuator can becontrolled using input from e.g., an acceleration sensor. Using the OIStechniques, DSLR cameras allow for 3-4 stops (equivalent to 8 to 16times) of longer hand-held exposure times. In mobile electronic devicessuch as smartphones, OIS based modules comprising highly miniaturizedactuators are also used for motion compensation and autofocus (AF). Forexample, the US patent U.S. Pat. No. 9,134,503B2 discloses aminiaturized lens actuator module developed for use in Apple iPhonecameras. The disclosed lens actuator module comprises an AF mechanismcapable of moving a lens for focus optimization and an OIS mechanismcapable of moving the lens for image stabilization Similar to OIS inDSLM cameras, this OIS mechanism is based on the voice-coil motor (VCM)technology.

Existing VCM based OIS techniques are limited to compensation oflow-amplitude passive vibrations (e.g., handshake vibrations) of ahand-held optical imaging device (e.g., camera) and thus are not capableof compensating any active movement of a stage module that is purposelyimplemented e.g., in a scatterometer metrology tool by following apredefined wafer sampling strategy. High-speed and high-accuracypositioners could be used to further reduce the MAM time and henceimprove the throughput. For many metrology applications such as overlaymetrology, it is desirable that a high-speed positioner can provide apositioning accuracy of, for example, better than 1 micrometer. However,high-speed positioning requires high-speed control electronics andhigh-accuracy positioning requires high precision actuators. Fulfillingsuch stringent requirements results in positioners becoming tooexpensive and bulky.

In this disclosure, methods and apparatuses are proposed to reduce theMAM time and hence to increase the throughput of a metrology orinspection tool in a more cost-effective manner. For the sake ofreadability often only metrology is mentioned. However, metrology,inspection tool and something similar are meant. The proposed methodssignificantly reduce the MAM time by enabling good quality images to beacquired while a positioner (and thus a sample) is still in motion,i.e., a sample is not stationary with respect to imaging optics of atool (e.g., objective lens, image sensor) during image acquisition. Theproposed methods and concepts are embodied in the various examplemetrology tools illustrated in FIGS. 8 to 11 . Note that for the sake ofsimplicity, all the figures comprise simplified schematics which showonly some components, e.g., those sufficient for the purpose ofdescribing the working principle of the proposed methods.

The embodiments may comprise a motion compensatory mechanism having adynamic mounting for a component of the optical imaging system andactuator to actuate a compensatory movement of the component.

FIG. 8 schematically illustrates an embodiment of an optical imagingsystem or scatterometer metrology tool where a high NA objective lens isused for motion compensation. As illustrated in FIG. 8 , an illuminationbeam of radiation IB may be reflected into an objective lens OB by aturning mirror TM. The illumination beam IB may comprise a beam diameterthat is smaller than the pupil diameter of the objective lens OB and maypass through the center of the objective lens OB. The illumination beamIB may be subsequently focused onto a target OT of a substrate WA. Thefocused illumination beam IB may comprise a spot diameter that isinversely proportional to the NA of the objective lens OB.

The substrate WA may be supported and positioned by a stage module SMwhich may comprise a substrate support (not shown) for holding thesubstrate WA. The (e.g., grating based overlay) target OT may diffractthe illumination beam IB into a number of diffraction orders. In thecase where the tool is configured for dark field imaging, the zerothdiffraction order may be blocked by an optical component (not shown). Inthis embodiment, two non-zeroth diffraction orders, e.g., positive firstdiffraction order DB1 and negative first diffraction order DB2, may becollected by the objective lens OB. In other embodiments, only onenon-zeroth diffraction order (e.g., positive first diffraction orderDB1) may be collected by the objective lens OB. The objective lens OBmay be configured to comprise a high NA, which may be greater than 0.7,greater than 0.8 or greater than 0.9, for example: in the range between0.7 and 0.97, or between 0.9 and 0.95. A high NA objective lens OB notonly improves optical resolution but also helps avoid spatialoverlapping between the diffracted beams DB1, DB2 (or more generallysignal beams) and the illumination beam IB in the pupil plane of theobjective lens OB, which is highly desired for obtaining good imagequality. In different embodiments (as described below), illumination anddetection may be separate using one or more objective lenses with lowerNA.

In the non-limiting embodiment of FIG. 8 , the objective lens OB maycomprise two lens elements or lens groups, i.e. a first lens element orlens group OE1 and a second lens element or lens group OE2. At least oneof the two lens elements or lens groups OE1, OE2 may be configured to bemoveable within the external casing of the objective lens OB. In thisembodiment, such movement may be limited in a plane (e.g., x-y plane)substantially perpendicular to the optical axis of the objective lensOB. In other embodiments, the lens elements or lens groups OE1 may beadditionally configured to move along the optical axis of the objectivelens OB such that fine optimization of the focusing of the objectivelens OB may be achievable. In other embodiments, the objective lens OBmay comprise any number (e.g., single or multiple) of lens elements orlens groups.

An image lens IL may be used to focus the diffracted beams DB1, DB2,onto an image sensor IS such that an image IM of the target OT isformed. In addition, one or more optical elements (not shown) may beused e.g., to shape and/or direct the diffracted beams DB1, DB2 at thepupil plane of the objective lens OB. The one or more optical elementsmay comprise for example one or more optical wedges for directing thediffracted beams to desired locations on the image sensor IS, or one ormore optical filters for selectively transmit desired wavelengths. Indifferent embodiments, there may be no image lens IL and/or otheroptical elements.

The stage module SM may comprise one or more actuators (e.g.,electromagnetic actuators) and may allow for movement with multiple(e.g., six) degrees of freedom. In the embodiment of FIG. 8 , theobjective lens OB unit and the image sensor IM may be fixed in position.In order to evaluate a parameter of interest (e.g., overlay error of anew process layer), measurements may be made over various targets OTthat are distributed across the substrate WA. The stage module SM may beconfigured to move the substrate WA between targets OT by following apredefined sampling scheme. The stage module SM may move in a planesubstantially parallel to the x-y plane (according to the coordinatesystem in FIG. 8 ). Note that, the proposed motion compensation methodsare not limited only for imaging a moving target/object. They areapplicable so long as there is relative motion between the target andthe objective lens OB unit during image acquisition. For example, indifferent configurations, the scatterometer metrology tool 800 maycomprise a stationary stage that supports the substrate WA in a fixedposition and a movable objective lens OB unit that is configured todirect the illumination beam IB to different parts of the substrate WA.In other configurations, the whole imaging section (comprising e.g., theobjective lens OB unit, the illumination lens IL, and the image sensorIM) of the scatterometer metrology tool 800 may be moveable with respectto a stationary sample. In all these cases, the proposed motioncompensation methods can be used to improve image quality andthroughput.

Referring back to FIG. 7(b), during an image acquisition period ΔT2′,the stage module SM may first decelerate from an intermediate speedV_(I) to a predefined minimum speed V_(min) and, immediately when theminimum speed V_(min) is reached or shortly thereafter, recommenceacceleration. In other embodiments, the stage module SM may maintain theminimum speed V_(min) for a short period of time before starting toaccelerate again. Subsequently, the stage module SM may continue toaccelerate until a predefined maximum speed V_(max) is reached. Thestage module SM may then move at the maximum speed for a period of timee.g., as determined by the distance between the current target OT andthe next target OT. When the next target OT is nearing the focusedillumination beam IB, the stage module SM may start to decelerate fromthe maximum speed V_(max) towards the minimum speed V_(min). At theintermediate speed V_(I), the stage module SM may have entered animaging zone where the focused illumination beam IB can measure the nexttarget OT. At this point of time, a further image acquisition can beperformed which takes the same amount of time ΔT2′ as the previous imageacquisition. The stage module SM may again begin to accelerate beforethe image acquisition is finished, e.g., such that the intermediatespeed V_(I) is reached at around the time the capture is completed. Notethat, the above-mentioned minimum speed, intermediate speed and maximumspeed can be determined according to the configuration of a metrologytool as well as application needs.

A moving target OT during image acquisition may cause the diffractedbeams DB1, DB2 to shift with respect to the image sensor IS andtherefore result in a spatial-shifting image on the image sensor IS.During an image exposure, such a spatial-shifting image may result in aglobal ‘motion blurring’ artefact in the direction of motion, which willlower the accuracy of the computed parameter of interest values withoutfurther measures. To address this problem, it is proposed to provide amotion compensatory mechanism which is able to compensate for the motioninduced the beam shifting and thus maintain the image position on theimage sensor IS.

In the embodiment of FIG. 8 , the objective lens OB may be configuredsuch that the first lens element or lens group OE1 is translatable withrespect to the second lens elements or lens groups OE2 or vice versa.For example, the first lens element or lens group OE1 may be translatedin a plane substantially perpendicular to the optical axis of theobjective lens OB, e.g., the x-y plane according to the coordinatesystem in FIG. 8 . The translation of the first lens element or lensgroup OE1 may be actuated using at least one actuator. The at least oneactuator may be located within the external casing of the objective lensOB. The at least one actuator may comprise for example at least oneminiaturized voice-coil actuator, or at least one miniaturizedelectromagnetic actuator.

The translation of the first lens element or lens group OE1 may achieveone or both of: 1) shifting the illumination beam IB such that it alwaysilluminates the same area of the moving target OT; and 2) compensatingfor the spatial shifting of the diffracted beams DB1, DB2 in the pupilplane of the objective lens resulting from the moving of the target OT.

FIGS. 9(a) and 9(b) respectively illustrates operating principle ofmotion compensation implemented in the first configuration of thescatterometer metrology tool in accordance with an embodiment. For thesake of simplicity, the stage module SM in FIGS. 9(a) and 9(b) are onlyshown moving in +x direction. In reality, the stage module SM may movein any direction within the x-y plane (according to the coordinatesystem in FIG. 8 ). An image acquisition may start at the first timeinstance T1 and finish at the second time instance T2. During theacquisition period, i.e. between the two time instances T1 and T2, thetarget OT may have moved by a distance of Δx along the +x direction.

As mentioned above, without any motion compensation measures, theacquired image IM will show motion blurring artefact resulting fromrelative movement between the illumination beam IB and the moving targetOT. So by translating the first lens element or lens group OE1 along thesame direction, i.e. +x direction, it is possible to ensure theillumination beam IB moves together with the overlay target OT in asubstantially synchronized manner such that the illumination beam IBalways illuminates the same area or substantially the same area of thetarget OT at least during the image acquisition period. The distance+Δx′ that the first lens element or lens group OE1 is required to movein order to sufficiently compensate motion (or to closely follow thetarget OT) may depend on the specific design of the objective lens OB.At the second time instance T2, since the second lens element or lensgroup is fixed in position, the shifting of the first lens element orlens group OE1 may result in the illumination beam IB being obliquelyincident on the overlay target OT. Oblique incidence of the illuminationbeam IB may in turn result in the diffracted beams DB1, DB2 propagatingalong different optical paths before being collected by the objectivelens OB. While transmitting through the lens elements or lens groupsOE1, OE2 within the objective lens OB, the spatial shifting of thediffracted beams may be substantially compensated by the lateralshifting of the first lens element or lens group OE1.

The overall effect of translating the first lens element or lens groupOE1 may be such that upon leaving the objective lens OB, the diffractedbeams DB1, DB2 may follow substantially the same optical paths that leadto substantially the same position on the image sensor IS at leastduring each image acquisition period. In other words, the image IMformed on the image sensor may stay substantially the same position andhave substantially the same sharpness even when an image is taken from amoving target OT.

The scatterometer metrology tool 800 may comprise a control unit CUconfigured to control some or all of the moveable parts or components inthe tool. The control unit CU may comprise one or more sub control units(not shown), each being configured to control one component in the tool.In the embodiment of FIG. 8 , the control unit CU may comprise a firstsub control unit configured for controlling the actuators of the stagemodule SM, a second sub control unit configured for controlling theactuators of the objective lens OB, and a third sub control unitconfigured for controlling the image sensor IS for image acquisition. Inother embodiments, the control unit CU may comprise one or more subcontrol units which may be respectively configured for different controltasks. The control unit CU may be a computer system and may comprise atleast one processor and one memory.

In the cases where a wafer or a sample is measured or inspected byfollowing a predefined sampling strategy/scheme, the control unit CU mayreceive the information detailed in the sampling scheme that is to beexecuted. Such sampling details may comprise for example thedistribution of the targets OT that are selected for measurement, thesampling order of each selected target OT (or the target motiontrajectory of the stage module SM), the maximum and minimum speeds ofthe stage module SM, the acceleration and deceleration rates of thestage module SM. The sampling details may provide the control unit CUwith the time-dependent movement vector of the stage module SM. Thecontrol unit CU may further input the predefined sampling details to aprediction model, which may be stored in the memory of the control unitCU and may be configured to predict the dynamic behavior of the stagemodule SM during the next moving operation, in particular the dynamicbehavior during the image acquisition period (e.g., ΔT2/ΔT2′ in FIG. 7). Having the knowledge of the time-dependent movement vector andpredicted dynamic behavior of the stage module SM, the control unit CUmay be able to real-time track the damped oscillatory settle behavior ofthe stage module SM using feedforward control. The control unit may beable to control the movement of the first lens element or lens group OE1in such a way that it moves substantially synchronously with the stagemodule SM at least during each image acquisition period. In someembodiments, additional sensors may be used to measure for examplereal-time position and moving parameters (e.g., speed, acceleration,etc.) of the stage module SM such that more accurate and robust motioncompensation can be obtained.

Referring back to FIGS. 9(a) and 9(b), when a new target OT is enteringthe imaging zone, which may be defined as the region between a firststage position at the first time instance T1 and a second stage positionat the second time instance T2, the control unit CU may command theactuators to move/translate the first lens element or lens group OE1 ina synchronized manner. The moving characteristics of the first lenselement or lens group OE1, such as for example moving speed,acceleration and deceleration rates, moving distance, may be determinedbased on, for example, the programmed movement vector, the predicteddynamic behavior of the stage module SM, as well as the design of theobjective lens OB (and the dynamic behavior of first lens element orlens group OE1). The design of the objective lens OB may determine amotion factor that defines the relationship between the movingcharacteristics of the stage module SM and those of the first lenselement or lens group OE1.

Once an image acquisition is complete, the control unit CU may configurethe actuators of the stage module SM for the next moving operationaccording to the sampling scheme. The control unit may command theactuators to move the stage module SM to the next predefined positionsuch that a next target OT can be measured/imaged. In the meantime, thecontrol unit CU may also configure the actuators of the first lenselement or lens group OE1 for the next predefined moving operationaccording to the programmed movement vector, the predicted dynamicbehavior of the stage module SM and the aforementioned motion factor.The first lens element or lens group OE1 will be translated or movedwhen the next target OT enters the imaging zone.

The forgoing embodiments are described in an example configuration wherea high NA objective lens is employed for both illumination anddetection; however, it should be appreciated that the proposed methodsare equally applicable for other configurations where one or more lowerNA objective lenses are used (e.g., the objective lens may be used onlyfor detection with a separate lens or illumination beam delivery systemused in the illumination branch).

FIG. 10 schematically illustrates an embodiment of an optical imagingsystem or the scatterometer metrology tool where a low NA objective lensis used for motion compensation. In the embodiment of FIG. 10 , the lowNA objective lens OB may comprise at least one lens element or lensgroup configured to be translatable and may be used solely fordetection.

With reference to FIG. 10 , an illumination rotating mirror IRM may beused in the embodiment to direct a (focused) illumination beam ofradiation IB towards a substrate WA. The illumination beam IB may beobliquely incident on a target OT of the substrate WA. The illuminationbeam IB may be focused by a second low NA objective lens (now shown)located before the illumination rotating mirror IRM. Upon interactingwith the target OT, the illumination beam IB may be diffracted intomultiple diffraction orders, among which two first diffraction ordersDB1, DB2 (corresponding respectively to +1^(st) diffraction order and−1^(st) diffraction order) may be collected by an objective lens OB′.The objective lens OB′ may comprise a low NA which may be in the rangebetween 0.2 and 0.7, or between 0.3 and 0.7 or between 0.3 and 0.5. Thelow NA objective lens OB′ may be configured in a similar manner as thehigh NA objective lens OB in FIGS. 8, 9 (a) and 9(b). The objective lensOB′ may comprise two lens elements or lens groups, i.e. a first lenselement or lens group OE1′ and a second lens element or lens group OE2′,at least one of which may be configured to be translatable or movable bymeans of at least one actuator.

In contrast to the embodiment of FIG. 8 , where the translation of thefirst lens element or lens group OE1 enables the illumination beam IB toilluminate substantially the same area of the overlay target and thediffracted beams DB1, DB2 to follow substantially the same optical pathsto the image sensor, the movement of the illumination beam TB and themovement of the collected diffracted beams DB1, DB2 in the embodiment ofFIG. 10 may be no longer coupled together. Instead, the movement of theillumination beam IB on the target OT may be achieved separately throughe.g., pitch rotation PR and yaw rotation YR of the illumination rotatingmirror IRM. In this example, the translation of the first lens elementor lens group OE1′ may be used for compensation of only the shifting ofthe collected diffracted beams DB1, DB2 to ensure a substantiallystationary image IM on the image sensor IS at least during each imageacquisition period. The spatial shifting of the diffracted beams DB1,DB2 may be resultant from the movement of the illumination beam IB whichchanges the incidence angle of the illumination beam IB with respect tothe target OT and thus the propagation directions of the diffractedbeams DB1, DB2. The rotation of the illumination rotating mirror IRM maycontrol the illumination beam IB to closely follow the moving target OTsuch that the illumination beam IB illuminates substantially the samearea of the overlay target OT at least during each image acquisitionperiod. The translation of the first lens element or lens group OE′, therotation of the illumination rotating mirror IRM, the movement of thestage module SM and the image acquisition of the image sensor IS may allbe controlled by a control unit CU′.

In the embodiment of FIG. 10 , the control unit CU′ may function in asimilar manner as that in the embodiment of FIG. 8 . The control unitCU′ may also comprise one or more sub control units, each beingconfigured to control one component in the tool. For example, thecontrol unit CU′ may comprise a first sub control unit configured forcontrolling the actuators of the stage module SM, a second sub controlunit configured for controlling the actuators of the objective lens OB,a third sub control unit configured for controlling the image sensor ISfor image acquisition, and a fourth sub control unit configured forcontrolling the rotation of the illumination rotating mirror IRM. Inother embodiments, the control unit CU′ may comprise one or more subcontrol units which may be respectively configured for different controltasks. The control unit CU′ may comprise at least one processor and onememory.

In the cases where a wafer or a sample is measured or inspected byfollowing a predefined sampling strategy/scheme, the control unit CU′ inFIG. 10 may also receive the information detailed in the sampling schemethat is to be executed. The control unit CU′ may further input thepredefined sampling details to a suitable prediction model to predictthe dynamic behavior of the stage module SM during the next movingoperation. Having the knowledge of the programmed movement vector for,and dynamic behavior of, the stage module SM, the control unit may beable to control the movement of the illumination rotating mirror IRM andthe first lens element or lens group OE1 in such a way that both arecontrolled substantially synchronously with the stage module SM.

Note that the illumination delivery method of this embodiment (e.g.,illumination rotating mirror IRM) is only an example and othercontrollable illumination delivery methods and arrangements may beemployed which are controllable synchronously with the stage module SMand objective lens OB′ or an element thereof.

In different embodiments, rather than by translating at least one lenselement or lens group in the objective lens OB, OB′, the spatialshifting of the diffracted beams DB1, DB2 may be compensated bytranslating the whole objective lens OB, OB′. Each of the high NAconfiguration (e.g., the embodiment of FIG. 8 ) and the low NAconfiguration (e.g., the embodiment of FIG. 10 ) may be adapted tosynchronously move the whole objective lens OB, OB′ for imaging anonstationary/moving microscopic object, rather than only an elementthereof. For both high NA and low NA configurations, the translation ofthe whole objective lens OB, OB′ may be enabled by actuation of theobjective lens by at least one actuator (e.g., voice-coil actuators,electromagnetic actuators), e.g., under control of the control unit CU,CU′. The translation of the whole objective lens OB, OB′ may result insubstantially the same effect in terms of e.g., image quality andthroughput as the translation of at least one lens element or lensgroup.

In case of a high NA configuration where the illumination and detectionare coupled and both enabled by a single high NA objective lens OB, thesynchronous translation of the whole objective lens OB may allow theillumination beam IB to closely follow the moving target OT such thatthe illumination beam IB illuminates substantially the same area of themoving target OT at least during each image acquisition period. In themeantime, the synchronous translation of the whole objective lens OB mayalso compensate for the spatial shifting of the collected diffractedbeams DB1, DB2 such that upon leaving the objective lens OB, thediffracted beams DB1, DB2 may follow substantially the same beam pathsleading to the substantially the same image position on the image sensorIS at least during each image acquisition period.

In case of a low NA configuration where the illumination and detectionare decoupled, the synchronous translation of the whole objective lensOB may function to ensure only the diffracted beams DB1, DB2 followsubstantially the same beam paths leading to the substantially the sameimage position on the image sensor IS at least during each imageacquisition period. A separate mechanism (e.g., the illuminationrotating mirror IRM in FIG. 10 ) may be employed to synchronously movethe illumination beam IB such that the illumination beam IB illuminatessubstantially the same area of the moving target OT at least during eachimage acquisition period.

Rather than translating either the lens element or lens group of theobjective lens OB′ or translating the whole objective lens OB′, thespatial shifting of the diffracted beams DB1, DB2 may be compensated bytranslating or moving at least one optical component located between theobjective lens OB′ and the image sensor IS. In some embodiments in thelow NA configuration, the at least one optical component may comprisefor example the image lens IL in FIG. 10 . The translation of the imagelens IL may result in a substantially stationary image IM on the imagesensor IS at least during each image acquisition period. In some otherembodiments in the low NA configuration, the spatial shifting of thediffracted beams DB1, DB2 may be not actively compensated. Instead, theimage sensor IS may be configured to be translatable such that the imagesensor IS closely follows the moving image IM resulting from theshifting of the diffracted beams DB1, DB2 at least during each imageacquisition period. In both these examples, the illumination beam IB maybe configured to closely follow the moving target OT e.g., by means ofan illumination rotating mirror IRM as employed in the embodiment ofFIG. 10 or otherwise. In some embodiments, two, more or all of theoptical components in the detection branch of the scatterometermetrology tool 1000 (e.g., comprising the objective lens OB, the imagelens IL and the image sensor IS in FIG. 10 ) may be moveable for motioncompensation when imaging a nonstationary target or (microscopic)sample.

FIG. 11 schematically illustrates an embodiment of an optical imagingsystem or scatterometer metrology tool where an illumination rotatingmirror and a detection rotating mirror are used for motion compensation.Different to the embodiment of FIG. 10 , the detection branch of theembodiment of FIG. 11 may be folded by a detection rotating mirror DRM.In the scatterometer metrology tool 1100, upon leaving the objectivelens OB″, the diffracted beams DB1, DB2 may be reflected onto the imagesensor IS via the detection rotating mirror DRM. In some embodiments inthe third configuration, an image lens IL may be additionally placedin-between the image sensor IS and the objective lens OB″ to bettercontrol the image size on the image sensor IS. In this embodiment, theillumination beam IB may be controlled and moved in the same way as inthe embodiment of FIG. 10 . The synchronous movement of the illuminationbeam IB may ensure the illumination beam IB illuminates substantiallythe same area of the target OT.

The movement of the illumination beam IB may inevitably change theincidence angle of the illumination beam IB with respect to the targetOT and consequently change the propagation directions of the diffractedbeams DB1, DB2. Once collected by the objective lens OB″, the diffractedbeams DB1, DB2 may follow different optical paths that lead to adifferent image position on the image sensor IS. In order to compensatefor such an image shift so as to maintain a substantially stationaryimage IM on the image sensor IS during at least each image acquisitionperiod, the detection rotating mirror DRM may be configured to rotatee.g., in both pitch PR and yaw YR directions. Similar to the operationof the illumination rotating mirror IRM, the rotation of the detectionrotating mirror DRM may also be in synchronization with the movement ofthe target OT. Such synchronization may be controlled based on apredefined sampling scheme and predicted dynamic behavior of the stagemodule SM, as described above. In this way, the image shift resultingfrom the diffracted beams DB1, DB2 propagating along different opticalpaths may be actively compensated by the rotation of the detectionrotating mirror DRM. As a result, the image IM may be controlledsubstantially stationary on the image sensor at least during each imageacquisition period. The movement of the illumination rotating mirrorIRM, the detection rotating mirror DRM, the stage module SM and theimage sensor IS may all be controlled by the control unit CU″.

As an alternative to using a detection rotating mirror DRM in anarrangement such as illustrated in FIG. 11 , a digital micromirrordevice (DMD) device may be used to apply a phase ramp to the diffractedbeams. A disadvantage of using a DMD is that the frame rate is low andthe light loss associated with the use of Lee holograms. The angulardeviation obtained from a DMD can be in the range of 4-5 degrees but therequired image displacement can be obtained by increasing the distanceto the sensor. An advantage of using a DMD device is that aberrationscan be corrected for at the same time as image stabilization. As thetarget moves under the lens, the field dependent aberrations can change.This means the target looks different for different field positions.Using a DMD, these changing aberrations can be compensated foradaptively.

Implementation of non-stop metrology using a Digital micro mirrordevice.

In some embodiments, an image processing algorithm may be additionallyapplied to further compensate for any image blurring artefact beforecomputing a parameter of interest value, thereby substantiallymaintaining the original accuracy of the overlay value while increasingthe throughput. The working principle of the image processing algorithmmay be described by following equation:

J(x,y)=I(x,y)*h(x,y)+n(x,y),  Eq. (1)

where the I(x, y) denotes the value of an image pixel at coordinate (x,y); h(x, y) denotes the motion blur kernel which is the result of theoptical point spread function and the motion trajectory of the stagemodule SM during image acquisition; n(x, y) denotes the sum of all noisecontributions on the image which may comprise for example Poisson shotnoise, dark noise, quantization noise from analog-to-digital conversion.

The total blur kernel h(x, y) may be computed from an earlier-measuredoptical point spread function of the imaging system, e.g., thescatterometer metrology tool 800, 1000, 1100, in combination with theknown target motion trajectory belonging to an observed image withmotion blur. The image processing algorithm may be configured to correctfor the motion-part of the blur kernel as well as the non-ideal opticalpoint spread. Once the total blur kernel h(x, y) is determined, theoriginal un-blurred image Î(x, y)≈I(x, y) may be estimated from anobserved motion-blurred image (x) and knowledge of the motion blurkernel h(x,y). The estimation may be achieved by using one of existingimage restoration approaches such as for example Wiener filtering, imagedeconvolution, constrained least-squares, and other iterative methods.Note that the restored image Î(x, y) may contain high-frequency noisewhich stems from the inversion of the (low-pass) blurring kernel h(x,y). To reduce the influence of such high-frequency noise, it isdesirable that signal extraction may involve taking the average value ofa region-of interest.

The image processing algorithm may be configured to adopt a data drivenmethodology and to use a deep convolutional neural network (CNN) as sucha network is well-suited for recognizing patterns and processing images.One approach may be to let the deep CNN produce a de-blurred image froman observed blurred input plus known blur kernel and then performfurther processing on a region of interest to compute overlay values fore.g., x and y. Another approach is to let the CNN compute the overlayvalues for e.g., x and y directly from the same inputs.

According to the first approach, with reference to FIG. 12 , for networktraining a large number of example clean “ground truth’ images I(x, y)is used where for each example the image is obtained from a conventionalmeasurement with zero-motion during exposure. Then, for each exampleimage I(x, y) a motion-blurred version J(x, y) is computed using asignal model as in Eq 1, or alternatively, actual blurred images areameasured for different speed profiles. The advantage of using computedimages is that a large dataset can be created easily which is beneficialfor network training. For the computed images, noise may be includedbased on a realistic camera noise model which includes camera shotnoise, camera dark noise, analog electronics noise, and quantizationnoise from analog-to-digital conversion. Per input example, manydifferent randomized motion blur kernels can be applied to cover allrelevant use cases. The blurred (and noisy) image may be input to a CNN,where the CNN may have as many inputs as there are pixels, where the CNNmay have one or multiple hidden layers, and where the CNN has an outputlayer which outputs the estimated corrected image I′(x, y).

Alternatively, according to the second approach, the CNN may have anoutput layer which outputs the predicted overlay values OVL′_x, OVL′_y,as shown in FIG. 13 . The predicted overlay values OVL′_x, OVL′_y may becompared to the actual overlay values OVL_x, OVL_y to obtain predictionerror PE. The actual overlay values may be computed based on a normalimage I_n(x, y) and a complementary image I_c(x, y). The motion blurredversions J_n(x, y), I_c(x, y) of both normal and complementary imagesare computed using the signal model as in Eq. 1 before being used in theCNN to predict overlay values OVL′_x, OVL′_y. Of course, the basicconcept is applicable to other parameters of interest.

The motion blur kernel h(x, y) is also input to the CNN so that theneural network can learn the relationship between the motion of thewafer stage and the image blurring artefact, as shown in FIGS. 12 and 13. Training is done to minimize the prediction error PE, where one ofmany well-known training algorithms and error criteria can be taken.

The image processing algorithm may be configured such that motion blurkernel h(x, y) is replaced by a vector containing time samples of themeasured stage velocity (both magnitude and direction) during imageacquisition. Alternatively, the vector may contain time samples of themeasured stage position (both x and y) during image acquisition.Alternatively, the CNN can operate in a so-calledGenerative-Adversarial-Network (GAN) mode. Here, the CNN will generate‘candidate’ unblurred images, which are then blurred with the knownkernel, and compared to the actual observed images. The networkparameters are trained such that the candidate image with the best matchwith the observed is returned. The advantage of this mode is that thetraining does not require ‘ground-truth’ unblurred images.

For inference (that is execution on new image data) and according to thefirst approach, the trained convolutional neural network (CNN) is usedas a predictor operating on a newly-acquired motion-blurred image J(x,y) and knowledge of the blurring kernel h(x, y) from knowledge of howthe wafer stage was moved, to compute an estimated unblurred image I′(x,y), as shown in FIG. 14 . This is done for both a normal and acomplementary image. The resulting unblurred image pair can then be usedby the signal extraction module to compute the parameter of interest.

Alternatively, for inference according to the second approach, thetrained CNN is used as a predictor operating on a newly-acquiredmotion-blurred image J(x, y) and knowledge of the blurring kernel h(x,y) (e.g., obtained from knowledge of how the wafer stage was moved), tocompute overlay values (OVL_x, OVL_y) or other parameter of interest forx and y directly, as shown in FIG. 15 .

The image processing algorithm may also be configured to take a sequenceof N images (instead of single image as described above) using anacquisition time which is N times smaller and use the N images incombination with the known stage motion trajectory to compute oneaverage image where each of the N images have been motion-compensatedbefore averaging. In this way a restored image is created (with littleblur due to short exposure times) which can be used for further signalextraction for overlay/parameter of interest. The individual images outof the series of N will have a low quality in terms of noise (due to thevery short exposure time), but the restored image will effectively havethe noise level corresponding to the total integration time of theseries. Moreover, if the frequency of the residual mechanicaloscillations during acquire time is known, it is possible to take the Nimages such that they are in-phase with the oscillation, thereby makingthe effect of the oscillation more similar between the N images.

Further embodiments of the present systems and methods are disclosed inthe subsequent list of numbered clauses:

1. An optical imaging system, comprising:

-   -   a stage module configured to support an object such that an area        of the object is illuminated by an illumination beam;    -   an objective lens configured to collect at least one signal        beam, the at least one signal beam originating from the        illuminated area of the object;    -   an image sensor configured to capture an image formed by the at        least one signal beam collected by the objective lens; and    -   a motion compensatory mechanism operable to compensate for        relative motion of the stage module with respect to the        objective lens during an image acquisition by causing a        compensatory motion of one or more of:    -   said objective lens or at least one optical element thereof;    -   said image sensor; and/or    -   an optical element comprised within a detection branch and/or        illumination branch of the optical imaging system.        2. An optical imaging system as defined in clause 1, wherein        said motion compensatory mechanism is operable such that, during        the image acquisition, the image is maintained at substantially        the same position on the image sensor despite any motion of the        stage module during said image acquisition.        3. An optical imaging system as defined in clause 1 or 2,        wherein the motion compensatory mechanism comprises:    -   a dynamic mounting for the entire objective lens enabling said        compensatory motion; and an actuator to actuate said        compensatory motion.        4. An optical imaging system as defined in clause 1 or 2,        wherein the motion compensatory mechanism comprises:    -   a dynamic mounting for one or more optical elements within the        objective lens enabling said compensatory motion; and    -   an actuator to actuate said compensatory motion.        5. An optical imaging system as defined in clause 3 or 4,        wherein the at least one actuator comprises one or more of: at        least one voice-coil actuator, at least one balance spring,        and/or at least one micro-electromechanical system (MEMS)        structure.        6. An optical imaging system as defined in clause 1 or 2,        wherein the motion compensatory mechanism comprises:    -   a dynamic mounting for said image sensor enabling said        compensatory motion; and    -   an actuator to actuate said compensatory motion.        7. An optical imaging system as defined in clause 1 or 2,        wherein the motion compensatory mechanism comprises:    -   an optical element comprised within a detection branch and/or        illumination branch and dynamic mounting therefor enabling said        compensatory motion; and    -   an actuator to actuate said compensatory motion.        8. An optical imaging system as defined clause 7, wherein the        optical element comprises an image lens for imaging the image on        the image sensor.        9. An optical imaging system as defined in clause 7, wherein the        optical element comprises a detection dynamic mirror and        actuator therefor or digital micromirror device enabling said        compensatory motion by control of reflection of the at least one        signal beam to the image sensor.        10. An optical system as defined in any of clauses 1 to 5, being        configured such that the illumination beam illuminates said area        of the object via said objective lens.        11. An optical system as defined in any of clauses 1 to 9,        further comprising an illumination beam delivery system operable        to controllably deliver said illumination beam to said object.        12. An optical imaging system as defined in clause 11, wherein        said illumination beam delivery system comprises an illumination        dynamic mirror configured to reflect the illumination beam to        the area of the object such that the illumination beam moves        substantially synchronously with the stage motion so as to        illuminate substantially the same area of the object during        image acquisition.        13. An optical imaging system as defined in any preceding        clause, further comprising a control unit configured to        determine said compensatory motion for said motion compensatory        mechanism.        14. An optical imaging system as defined in clause 13, wherein        the control unit is configured to determine said compensatory        motion based on at least a control signal for control of the        stage module.        15. An optical imaging system as defined in clause 14, wherein        the control unit is further configured to determine said        compensatory motion on a predicted dynamic behavior of the stage        module during image acquisition.        16. An optical imaging system as defined in clause any of        clauses 13 to 15, wherein the control unit is configured to        control said stage module such that said image acquisition        commences prior to said stage module becoming stationary and/or        said sage module begins motion to perform a subsequent action        prior to completion of said image acquisition.        17. An optical imaging system as defined in any preceding        clause, wherein said control unit is operable to perform image        processing said image so as to remove any motion blurring        artefacts caused by any stage motion not compensated by said        motion compensatory mechanism by estimating an de-blurred image        from the acquired image and a predetermined motion blur kernel.        18. An optical imaging system as defined in clause 17, wherein        said blur kernel is determined from an earlier-measured optical        point spread function of the optical imaging system in        combination with a known object motion trajectory belonging to        an observed image with motion blur.        19. An optical imaging system as defined in clause 18, wherein        said control unit is configured to correct for a motion-part of        the blur kernel and a non-ideal optical point spread function.        20. An optical imaging system as defined in any of clauses 17 to        19, wherein said control unit is configured to use an iterative        image restoration algorithm to estimate said de-blurred image        from the acquired image and a predetermined motion blur kernel        21. An optical imaging system as defined in any of clauses 17 to        19, wherein said control unit is configured to use a trained        artificial intelligence model to estimate said de-blurred image        from the acquired image and a predetermined motion blur kernel.        22. A method for imaging an object using an optical imaging        system, comprising:    -   illuminating an area of the object with an illumination beam;    -   collecting at least one signal originating from the illuminated        area of the object during an acquisition period during at least        a portion of which said object is non-stationary;    -   acquiring an image from the at least one signal beam on an image        sensor; and    -   performing a compensatory motion of an optical element of the        optical imaging system during said acquisition period to        compensate for relative motion of the object with respect to an        objective lens module used to collect the at least one signal        during the acquisition period such that the image is maintained        at substantially the same position on the image sensor during        the acquisition period.        23. A method as defined in clause 22, wherein performing a        compensatory motion of an optical element comprises performing a        compensatory movement of the objective lens module or an optical        element comprised therein.        24. A method as defined in clause 22, wherein performing a        compensatory motion of an optical element comprises performing a        compensatory movement of said image sensor.        25. A method as defined in clause 22, wherein performing a        compensatory motion of an optical element comprises performing a        compensatory movement of one or both of:    -   an image lens used to image the image on the image sensor; or    -   a detection dynamic mirror used to reflect the at least one        signal beam to the image sensor.        26. A method as defined in clause 22, wherein performing a        compensatory motion of an optical element comprises performing a        compensatory control of a digital micrometer device.        27. A method as defined in clause 26, comprising controlling the        digital micrometer device to correct for aberrations        simultaneously with said motion compensation.        28. A method as defined in clause 22 or 23, comprising        illuminating said area of the object with the illumination beam        via said objective lens.        29. A method as defined in any of clauses 22 to 27, comprising        controlling the illumination beam such that the illumination        beam moves substantially synchronously with the object motion so        as to illuminate substantially the same area of the object        during image acquisition.        30. A method as defined in any of clauses 22 to 29, comprising        determining said compensatory motion based on at least a control        signal for control of a stage module used to transport said        object.        31. A method as defined in clause 30, comprising modeling the        dynamic behavior of the stage module; and    -   determining said compensatory motion based on the predicted        dynamic behavior of the stage module during image acquisition.        32. A method as defined in clause 30 or 31, comprising        controlling said stage module such that said acquisition period        commences prior to said stage module becoming stationary and/or        said sage module begins motion to perform a subsequent action        prior to completion of said acquisition period.        33. A method as defined in any of clauses 22 to 32, further        comprising performing image processing on said image so as to        remove any motion blurring artefacts caused by any object motion        by estimating an de-blurred image from the acquired image and a        predetermined motion blur kernel.        34. A method as defined in clause 33, wherein said blur kernel        is determined from an earlier-measured optical point spread        function of the optical imaging system in combination with a        known object motion trajectory belonging to an observed image        with motion blur.        35. A method as defined in clause 34, comprising correcting for        a motion-part of the blur kernel and a non-ideal optical point        spread function.        36. A method as defined in any of clauses 33 to 35, comprising        using an iterative image restoration algorithm to estimate said        de-blurred image from the acquired image and a predetermined        motion blur kernel.        37. A method as defined in any of clauses 33 to 35, comprising        using a trained artificial intelligence model to estimate said        de-blurred image from the acquired image and a predetermined        motion blur kernel.        38. A metrology device comprising an optical system as defined        in any of clauses 1 to 21.        39. A metrology device as defined in clause 38, comprising a        scatterometer metrology apparatus, a level sensor or an        alignment sensor.        40. An optical inspection device comprising an optical system as        defined in any of clauses 1 to 21.

Note that all the concepts disclosed herein are equally applicable toany metrology tool for which it may be beneficial to image a movingsample. Such a sample may comprise any metrology target, such as used inpost-processing metrology (e.g., overlay target, focus target, criticaldimension or any other structural dimension target) and/or used prior toprocessing (e.g., an alignment mark). Any such target may be a dedicatedtarget formed for the purpose of metrology, and/or actual productstructures. The metrology tool may be of a type such as illustratedschematically in FIGS. 4 to 6 , for example.

FIG. 16 is a block diagram that illustrates a computer system 1200 thatmay assist in implementing the methods and algorithm disclosed herein.Computer system 1200 includes a bus 1202 or other communicationmechanism for communicating information, and a processor 1204 (ormultiple processors 1204 and 1205) coupled with bus 1202 for processinginformation. Computer system 1200 also includes a main memory 1206, suchas a random access memory (RAM) or other dynamic storage device, coupledto bus 1202 for storing information and instructions to be executed byprocessor 1204. Main memory 1206 also may be used for storing temporaryvariables or other intermediate information during execution ofinstructions to be executed by processor 1204. Computer system 1200further includes a read only memory (ROM) 1208 or other static storagedevice coupled to bus 1202 for storing static information andinstructions for processor 1204. A storage device 1210, such as amagnetic disk or optical disk, is provided and coupled to bus 1202 forstoring information and instructions.

Computer system 1200 may be coupled via bus 1202 to a display 1212, suchas a cathode ray tube (CRT) or flat panel or touch panel display fordisplaying information to a computer user. An input device 1214,including alphanumeric and other keys, is coupled to bus 1202 forcommunicating information and command selections to processor 1204.Another type of user input device is cursor control 1216, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to processor 1204 and for controllingcursor movement on display 1212. This input device typically has twodegrees of freedom in two axes, a first axis (e.g., x) and a second axis(e.g., y), that allows the device to specify positions in a plane. Atouch panel (screen) display may also be used as an input device.

One or more of the methods as described herein may be performed bycomputer system 1200 in response to processor 1204 executing one or moresequences of one or more instructions contained in main memory 1206.Such instructions may be read into main memory 1206 from anothercomputer-readable medium, such as storage device 1210. Execution of thesequences of instructions contained in main memory 1206 causes processor1204 to perform the process steps described herein. One or moreprocessors in a multi-processing arrangement may also be employed toexecute the sequences of instructions contained in main memory 1206. Inan alternative embodiment, hard-wired circuitry may be used in place ofor in combination with software instructions. Thus, the descriptionherein is not limited to any specific combination of hardware circuitryand software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 1204 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 1210. Volatile media include dynamic memory, such asmain memory 1206. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 1202.Transmission media can also take the form of acoustic or light waves,such as those generated during radio frequency (RF) and infrared (IR)data communications. Common forms of computer-readable media include,for example, a floppy disk, a flexible disk, hard disk, magnetic tape,any other magnetic medium, a CD-ROM, DVD, any other optical medium,punch cards, paper tape, any other physical medium with patterns ofholes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer can read.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 1204 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 1200 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 1202 can receive the data carried in the infrared signal andplace the data on bus 1202. Bus 1202 carries the data to main memory1206, from which processor 1204 retrieves and executes the instructions.The instructions received by main memory 1206 may optionally be storedon storage device 1210 either before or after execution by processor1204.

Computer system 1200 also preferably includes a communication interface1218 coupled to bus 1202. Communication interface 1218 provides atwo-way data communication coupling to a network link 1220 that isconnected to a local network 1222. For example, communication interface1618 may be an integrated services digital network (ISDN) card or amodem to provide a data communication connection to a corresponding typeof telephone line. As another example, communication interface 1218 maybe a local area network (LAN) card to provide a data communicationconnection to a compatible LAN. Wireless links may also be implemented.In any such implementation, communication interface 1218 sends andreceives electrical, electromagnetic or optical signals that carrydigital data streams representing various types of information.

Network link 1220 typically provides data communication through one ormore networks to other data devices. For example, network link 1220 mayprovide a connection through local network 1222 to a host computer 1224or to data equipment operated by an Internet Service Provider (ISP)1226. ISP 1226 in turn provides data communication services through theworldwide packet data communication network, now commonly referred to asthe “Internet” 1228. Local network 1222 and Internet 1228 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 1220 and through communication interface 1218, which carrythe digital data to and from computer system 1200, are exemplary formsof carrier waves transporting the information.

Computer system 1200 may send messages and receive data, includingprogram code, through the network(s), network link 1220, andcommunication interface 1218. In the Internet example, a server 1230might transmit a requested code for an application program throughInternet 1228, ISP 1226, local network 1222 and communication interface1218. One such downloaded application may provide for one or more of thetechniques described herein, for example. The received code may beexecuted by processor 1204 as it is received, and/or stored in storagedevice 1210, or other non-volatile storage for later execution. In thismanner, computer system 1200 may obtain application code in the form ofa carrier wave.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

1.-15. (canceled)
 16. An optical imaging system, comprising: a stagemodule configured to support an object such that an area of the objectis illuminated by an illumination beam; an objective lens configured tocollect at least one signal beam, the at least one signal beamoriginating from the area of the object; an image sensor configured tocapture an image formed by the at least one signal beam collected by theobjective lens; and a motion compensatory mechanism operable tocompensate for relative motion of the stage module with respect to theobjective lens during an image acquisition by causing a compensatorymotion of one or more of: the objective lens or at least one opticalelement thereof; the image sensor; and/or an optical element comprisedwithin a detection branch and/or illumination branch of the opticalimaging system.
 17. The optical imaging system of claim 16, wherein themotion compensatory mechanism is operable such that, during the imageacquisition, the image is maintained at substantially a same position onthe image sensor despite any motion of the stage module during the imageacquisition.
 18. The optical imaging system of claim 16, wherein themotion compensatory mechanism comprises: a dynamic mounting for one ormore optical elements within the objective lens configured to allow thecompensatory motion; and an actuator configured to actuate thecompensatory motion.
 19. The optical imaging system of claim 16, whereinthe motion compensatory mechanism comprises: a dynamic mounting for theimage sensor enabling the compensatory motion; and an actuatorconfigured to actuate the compensatory motion.
 20. The optical imagingsystem of claim 16, wherein the motion compensatory mechanism comprises:an optical element comprised within a detection branch and/orillumination branch and dynamic mounting therefor enabling thecompensatory motion; and an actuator configured to actuate thecompensatory motion.
 21. The optical system of claim 16, furthercomprising: an illumination beam delivery system operable tocontrollably deliver the illumination beam to the object and theillumination beam deliver system comprising an illumination dynamicmirror configured to reflect the illumination beam to the area of theobject such that the illumination beam moves substantially synchronouslywith the stage motion so as to illuminate substantially the same area ofthe object during image acquisition.
 22. The optical imaging system ofclaim 16, further comprising a control unit configured to determine thecompensatory motion for the motion compensatory mechanism.
 23. Theoptical imaging system of claim 22, wherein the control unit isconfigured to determine the compensatory motion based on at least acontrol signal for control of the stage module.
 24. The optical imagingsystem of claim 23, wherein the control unit is further configured todetermine the compensatory motion on a predicted dynamic behavior of thestage module during image acquisition.
 25. A method comprising:illuminating an area of an object with an illumination beam; collectingat least one signal originating from the area of the object during anacquisition period during at least a portion of that the object isnon-stationary; acquiring an image from the at least one signal beam onan image sensor; and performing a compensatory motion of an opticalelement of the optical imaging system during the acquisition period tocompensate for relative motion of the object with respect to anobjective lens module used to collect the at least one signal during theacquisition period such that the image is maintained at substantiallythe same position on the image sensor during the acquisition period. 26.The method of claim 25, further comprising controlling the illuminationbeam such that the illumination beam moves substantially synchronouslywith the object motion so as to illuminate substantially a same area ofthe object during image acquisition.
 27. The method of claim 25, furthercomprising determining the compensatory motion based on at least acontrol signal for control of a stage module used to transport theobject.
 28. The method of claim 27, further comprising: modeling thedynamic behavior of the stage module; and determining the compensatorymotion based on the predicted dynamic behavior of the stage moduleduring image acquisition.
 29. A metrology device comprising: a stagemodule configured to support an object such that an area of the objectis illuminated by an illumination beam; an objective lens configured tocollect at least one signal beam, the at least one signal beamoriginating from the area of the object; an image sensor configured tocapture an image formed by the at least one signal beam collected by theobjective lens; and a motion compensatory mechanism operable tocompensate for relative motion of the stage module with respect to theobjective lens during an image acquisition by causing a compensatorymotion of one or more of: the objective lens or at least one opticalelement thereof; the image sensor; and/or an optical element comprisedwithin a detection branch and/or illumination branch of the opticalimaging system.
 30. An optical inspection device comprising a stagemodule configured to support an object such that an area of the objectis illuminated by an illumination beam; an objective lens configured tocollect at least one signal beam, the at least one signal beamoriginating from the area of the object; an image sensor configured tocapture an image formed by the at least one signal beam collected by theobjective lens; and a motion compensatory mechanism operable tocompensate for relative motion of the stage module with respect to theobjective lens during an image acquisition by causing a compensatorymotion of one or more of: the objective lens or at least one opticalelement thereof; the image sensor; and/or an optical element comprisedwithin a detection branch and/or illumination branch of the opticalimaging system.